A gas turbine is comprised of a compressor section for compressing ambient air, a combustor section for mixing and burning fuel and the compressed air, and a turbine section driven by the expanding combustion gases for powering the compressor and turning an output shaft for ancillary devices or power generation or for providing thrust for propelling an aircraft. In an aircraft application, one of these ancillary devices may be a large turbofan used to provide additional thrust for the aircraft.
Modern gas turbine engines require very large quantities of atmospheric air. Entrained in the air is particulate matter of various sizes and made up of many different substances, such as aerosolized salts, volatile organic compounds, smog particulate, dust, agricultural debris, pollen, hydrocarbons, insects, de-icing chemicals, and the like. Stationary gas turbines such as those used in power generation utilize air filtering mechanisms to remove the majority of the particulates. Even with this filtering, however, very small contaminants and foreign matter dissolved in the air humidity still remain in the airflow. Gas turbines on aircraft do not have any filtering system on the airflow—so all of the particulate matter enters the turbine. The majority of large particulate matter and debris entering an aircraft engine are centrifuged away from the gas turbine through the action of the turbofan; however some of this large particulate matter and debris may remain on the turbofan blades reducing their effectiveness. While most of the smaller particulate matter follows the gas path and becomes part of the engine exhaust, some of the particles will remain on the intake, compressor and gas path components of the engine. This remaining particulate matter is referred to as “fouling,” and after a period of operation will build up on the engine components. Any large debris that is ingested through the turbofan, such as insects and birds, can also cause considerable immediate fouling of the intake and gas path components.
The fouling on the turbofan and compressor blades of the turbine causes a loss in the aerodynamic properties of these components. The air stream is deflected by the increased roughness of the blade surfaces, and the boundary layer of the air stream is thickened resulting in a negative vortex at the trailing edge of the blade. The deflection and vortex results in a loss of air mass flow through the engine thereby reducing engine performance. Secondary effects of this fouling include reduced compressor pressure gain, which leads to the engine operating at a reduced pressure ratio, and a reduction in the compressor isentropic efficiency. The overall effect of these negative effects, all caused by turbofan and compressor blade contamination, is that the engine will utilize more of the power it is generating to turn the compressor rather than producing thrust (in the case of an aircraft) or powering other ancillary systems (such as generators in a power generation application).
In an aircraft application, it is necessary to increase the throttle for more power to compensate for the lost thrust. In a stationary application, it is necessary to increase fuel flow to make up for the lost power. In both cases, operating costs increase as more fuel is required to perform the same amount of work. The increased fuel burn, in addition to increasing operating costs, has other deleterious effects:
First, the loss in performance caused by fouling causes a rise in the temperature at which the engine operates. This is most critical in aircraft engines since they operate at temperatures very close to the limit of what the engine material can withstand. The temperature is measured after combustion and is designated as the Exhaust Gas Temperature (EGT). The difference between the actual EGT and the safe operating limit established by the engine manufacturer is the EGT Operating Margin. As fouling takes place over time, the EGT rises to a point closer to the upper limit of the EGT Operating Margin. When the EGT is too close to the EGT Operating Margin, the engine must be removed from service and overhauled. In stationary power generation applications, the increase in engine operating temperature reduces the serviceable life of the gas turbine due to early component failure.
Second, fouling of the engine is harmful for the environment. Increased fuel burn increases emissions of gases known to cause global warming such as carbon dioxide. Each additional pound of fuel burned results in the emission of approximately 3.4 pounds of carbon dioxide. Increased combustion temperatures due to fouling also results in increased nitrogen oxides (NOx) known to cause smog.
In addition to the above described intake component fouling effects, fouling in the combustor and other post-compressor areas has negative effects on the engine. Debris in the air stream carried into the rear section of the engine increases the amount of ash and soot in the combustor, reducing its effectiveness and leading to potential corrosion of components.
Gas turbines, both in aircraft and stationary applications, can have many different designs but the above-described problem of airborne contaminants fouling the engine is common to all of them. There is a difference, however, in the types of fouling that occurs in the different sections of the engine. Most modern aircraft have a turbofan engine that is designed to provide thrust. The engine is comprised of a core fuel-burning gas turbine that drives a bypass fan. The fan is located upstream of the compressor and may be as large as 120 inches or more in diameter. The air being ingested into the engine first comes into contact with this bypass fan, and a portion of the airflow (known commonly as the “bypass ratio”) is then forced under very high velocity around the core of the engine and out through the rear of the engine producing most of the thrust utilized to propel the aircraft. Due to centrifugal and inertia! forces, most of the large particulates (including insects, bird residue, large dirt particles, etc.) in the air follow this path and subject the fan to fouling. Cleaning this fan is performed by a liquid cleaning medium generally using some form of impingement.
Downstream of the bypass fan is the core engine compressor. A core engine compressor is a component of all gas turbines (unlike the bypass fan which is only found on aircraft engines). The purpose of the compressor is to compress the air to high pressure ratios for maximum combustion efficiency. Compressing the air takes place: in a series of rows of decreasing size rotating blades and stationary vanes. The compression of the air leads to high pressure ratios and concurrent high temperatures that may exceed 900 degrees Fahrenheit. Most of the large particles of potential contaminants trapped in the airflow, as stated above, have either been filtered out through the use of filtering media or, in the case of turboaircraft, been centrifuged within the bypass fan area. Most of the remaining contaminants are very small particles in the airflow and other matter (such as aerosolized salts, for example) entrained in the humidity of the airflow. As the air is accelerated before the first stages of the compressor, according to the Bernoulli Effect, there is a corresponding initial temperature drop. This may cause some of the humidity to condense freeing the matter dissolved in the humidity. A portion of this freed matter strikes and adheres to the first rows of the compressor. The remaining freed matter mixes with the other fine particulate in the airflow. As the airflow continues through the compressor and the air is heated, this matter is subject to continued adhesion to the compressor blades and other gas path components causing additional fouling. The increasing heat may cause the fouling to be baked on to the blades. Typical fouling in the compressor section of gas turbines is salt, hydrocarbons, chemical residue, and other fine matter. This fouling is different than that generally found on the bypass blades and as such needs a different cleaning methodology. Where the bypass cleaning is accomplished by impingement of the cleaning medium on the bypass blades, the compressor cleaning is accomplished by reversing the process whereby the fouling was created in the first place. This process reversal is to re-entrain the particulate and fouling matter in a liquid medium which is then carried through the compressor and gas path and out the engine exhaust.
Removal of the aforedescribed contaminants from blades and vanes of in-service compressors is desirable to restore compressor and engine efficiency. Since it is both time-consuming and expensive to disassemble the engine from the aircraft and then the compressor from the engine, it is also desirable to remove the contaminants while the engine is on-wing. Furthermore, any method utilized to remove the contaminants must not interfere with the structural or metallurgical integrity of other components of the engine. In this regard, it is known in the art that contaminants can be removed from the internal components of a gas turbine engine by ingesting, into the engine inlet at dry-motoring or idle speed, substances generally characterized as aqueous cleaning compositions.
One of the early leaders in this field of cleaning aircraft engines is Juniper Aircraft Service Equipment of Liverpool, England. In the 1960's, they produced and patented one of the first gas turbine compressor washing units under U.S. Pat. No. 3,335,916. This patent discloses a mobile spray unit for use in the washing of compressor blades of gas turbine engines. The washing liquid is supplied under pressure through the use of a hand pump or through pressurized air provided by an external engine compressor connected to an air inlet to the unit. A flexible hose delivers the washing liquid from the mobile spray unit to the gas turbine engine. Improvements to this type of device have occurred, such as the mobile cart-mounted unit described in U.S. Pat. No. 4,059,123 to Bartos, et al. In contrast to the Juniper device described above, pressurization is achieved by the use of an integrally mounted air compressor driven by an internal combustion engine. In both of these devices, and others of a similar design using hand pumps or air pressure systems, the water pressure achieved is insufficient to atomize the washing liquid into a spray fine enough to get complete gas path penetration without having the liquid subjected to the centrifugal forces of the rotating components. A high pressure device described in U.S. Pat. No. 5,868,860 to Peter Asplund describes a wash system improvement that sought to improve on the existing low pressure systems by utilizing a high pressure electrically powered pump. This system specifies delivering the wash fluid at a pressure in excess of 50 bar (725 psi) in order to more finely atomize the wash fluid. The problem with this system is that the pressures utilized by this system are such that damage to the engine components may occur.
Many prior art methods exist for directing the washing liquid from the mobile units described above to the compressor components of the aircraft engines. For example, U.S. Pat. No. 4,196,020 to Hornak, et al. discloses a typical engine wash spray apparatus that is releasably connected to the leading edge of a gas turbine engine for dispersing a cleaning or rinsing fluid to the air intake area of the engine and on into the engine's internal air flow path. The apparatus includes a manifold having a plurality of spray nozzles symmetrically disposed about the air intake of a combustion turbine engine. Water is sprayed under pressure from these nozzles into the inlet of the compressor during operation. The inlet air is used to carry the atomized water mist through the turbine. A similar system is disclosed by McDermott in U.S. Pat. No. 5,011,540. The McDermott patent discloses a manifold having a plurality of nozzles for mounting in front of the air intake of a combustion turbine. McDermott proposes that a cleaning solution be injected into the air intake as a cloud dispersed in the less turbulent air found at the periphery of the intake. McDermott asserts that dispersal in the less turbulent air improved cleaning. In each of these and all other spray apparatus art the nozzles being used on the manifold each deliver the cleaning fluid at the same flowrate and spray droplet size. This fails to account for the different types of fouling occurring on the different components of the engine, and water flowrate and spray droplet size needed to address these differences.
The wash water and engine effluent is often collected for treatment, rather than being expelled directly onto the underlying surface (such as the hangar floor). One such system is described in U.S. Pat. No. 5,899,217 to Testman. Testman describes an engine wash recovery system that is temporarily installed on an aircraft turbine engine to recover wash liquids and contaminants washed from the engine during engine cleaning operations. The apparatus basically comprises a collector, an engine exhaust duct, and a container. The collector is formed of a flexible, liquid-proof material which is removably secured beneath the engine housing of the aircraft, to capture wash liquids which spill from the housing. The duct is removably connected to the engine exhaust, to capture spray which has passed through the engine as the engine is turned over at a relatively low rpm to flush the fluid through the engine. Both the collector and the duct are connected to a container for recovering all liquids emanating from the engine and its housing during the wash process. The collector may include a forward extension to capture liquids which back up out of the engine intake or inlet, and a rearward extension which extends back to the exhaust. A more involved collector system is seen in US Patent Application Publication No. US-2003/0209256-A1 which discloses a large jacket, wet suit, or cover that completely encases a turbojet or turbofan engine and a drum or 55 gallon barrel interconnected by hoses creating a jet engine cleaning system that confines the spraying, filtering, and collecting of chemical spray and toxic runoff to a confined system. These and all other collector systems are passive in nature; that is, they rely on the low-RPM exhaust of the engine to move the effluent. Very low engine RPM may cause the effluent spray to be too weak to enter or penetrate the collector or mist eliminator. At higher RPMs, any pressure drop or resistance along this path may lead to effluent being expelled around or in front of the collector or through leaks in the hoses or apparatus, or backed up in the tubing/hoses with resultant spillage.
Accordingly, there is a need for improved methods and apparatuses for cleaning gas turbine engines and collecting the resultant effluent to eliminate the constraints, limitations, and design drawbacks seen in the prior art and described above.